Various approaches have been tried for monitoring the blood pressure of living subjects. One approach is to insert a pressure sensor directly into a suitable artery in the subject. The sensor can be Connected to a suitable monitoring device by a lead which passes through the subject's skin. This approach provides accurate and instantaneous blood pressure measurements. A disadvantage of this approach is that it is invasive. A surgical procedure is required to introduce the pressure sensor. The fistula through which the lead exits the subject's body can provide a pathway for infection.
Another approach to measuring blood pressure uses a sphygmomanometer. A typical sphygmomanometer has an occluding cuff capable of being wrapped around a subject's arm; a pump for inflating the cuff; either an aneroid or mercury gravity sphygmomanometer to measure pressure in the cuff; and a stethoscope or other system for detecting Korotkoff sounds. Such devices are widely used in hospitals and doctors' offices for making routine blood pressure measurements but are not well adapted to providing continuous blood pressure monitoring.
Another method for measuring blood pressure is the oscillometric method. Oscillometric blood pressure measurements are made by using a transducer to detect and measure pressure waves in a pressure cuff as blood surges through an artery constricted by the pressure cuff. Many currently available digital blood pressure monitors use the oscillometric method for determining blood pressure. The oscillometric method is not ideal for continuous blood pressure monitoring because it typically cannot produce an updated blood pressure reading more frequently than about once every 30 seconds. Further, the cuff compresses underlying tissues. Over an extended period of time this can cause tissue damage.
There has been significant research directed toward the development of new non-invasive techniques for monitoring blood pressure. One approach exploits the correlation between blood pressure and the time taken for a pulse to propagate from a subject is heart to a selected point on a subject's artery. This approach is possible because the speed at which pulse waves travel from the heart to points downstream in a subject's blood circulatory system varies with blood pressure. As blood pressure rises the propagation velocity of arterial pulse waves increases and the pulse transit time decreases. In general, such methods may be called Pulse Transit Time (or “PTT”) methods. Typically a signal from an electrocardiogram (EKG) is used to detect a heart best and a pressure sensor is used to detect the arrival of a pulse wave generated by the heart beat at a downstream location. This approach is described, for example, by Inukai et al., U.S. Pat. No. 5,921,936. The Inukai et al. system uses an electrocardiogram to detect the start of a heart beat and uses a cuff equipped with a pressure sensor to detect pulse waves. Other similar systems are described in Orr et al., European Patent application No. EP0181067. A variation of this approach is described in Golub, U.S. Pat. No. 5,87,975.
One difficulty with PTT blood pressure measurement systems which measure blood pressure as a function of the time between the pulse of an EKG signal and a detected pulse wave is that there is a delay between the onset of an EKG pulse and the time that the heart actually begins to pump blood. This delay can vary significantly in a random way, even in healthy subjects. Hatschek, U.S. Pat. No. 5,309,916 discloses a method for measuring blood pressure by determining the time taken for a pulse to propagate downstream along a single arterial branch. This approach eliminates uncertainties caused by the imperfect correlation between EKG signals and the delivery of blood by the heart. However, it has the disadvantage that it can be difficult to arrange two sensors so that they can detect a pulse at each of two widely spaced apart locations along a single arterial branch.
Another difficulty with prior art PTT blood pressure measurements is that the relationship between blood pressure and the time taken for pulses to transit a portion of the blood circulatory system is different for every subject. Thus, it is necessary to calibrate a ITT blood pressure measurement system for each subject.
The book entitled Monitoring in Anesthesia and Critical Care Medicine, 3rd Edition, edited by Blitt and Hines, Churchill Livingstone, 1995, mentions a blood pressure monitor having the trade name, ARTRAC™ 7000 which used two photometric sensors, one on the ear and another on a finger, to measure diastolic blood pressure. This device apparently used the difference in arrived times of pulses at the ear and finger to measure the pulse transit time. The diastolic pressure was estimated based on a relationship of pressure and pulse wave velocity. This device apparently computed systolic pressure from the pulse volume. Further information about this device is provided in a FDA 510 (k) Notification entitled, “ARTRAC™ Vital Sign Monitor, Models 7000 and 5000 (K904888),” submitted by Sentinel Monitoring, Inc., 1990.
A relationship between blood pressure and pulse transit time can be developed by assuming that an artery behaves as if it were a thin-walled elastic tube. This relationship, which is known as the Moens-Korteweg-Hughes equation is described in more detail below. The Moens-Korteweg-Hughes equation depends on the elasticity and geometry of blood vessels and is highly nonlinear.
Inventors Aso et al., U.S. Pat. No. 5,564,427, proposed the use of a linear equation to calculate blood pressure using the EKG based pulse transit time. This method was further developed by Hosaka et al., U.S. Pat. No. 5,649,543. To calibrate the linear measurement system, Sugo et al., U.S. Pat. No. 5,709,212, introduced a multi-parameter approach to determine the parameters at deferent blood pressure levels for systolic and diastolic pressures respectively. Shirasaki patented another method to calibrate the parameters based on the multiple blood pressure reference inputs in Japanese patent No. 10-151118.
Despite progress that has been made in the field of blood pressure measurement, there remains a need for devices for blood pressure measurement which have acceptable accuracy and do not require complicated calibration steps.